The present invention generally relates to S-alkylthiopropionic acids which are esterified with pentaerythritol or the like, as well as to methods for making same. More particularly, the invention relates to so-called mixed esters. The mixed esters contain multiple 3-alkylthiopropionic acid groups, at least two of the alkyl chains thereof being of different carbon chain lengths. These mixed esters are useful as stabilizers of organic materials such as polymer resins and the like, and the mixed esters can be tailored to provide properties that are especially suitable for specific stabilizer needs.
Alkyl esters derived from alkylthioalkanoic acids and the like are, in general, known to be useful as stabilizers of organic materials such as polymer resins and the like which are otherwise subject to thermal and oxidative deterioration during processing, extrusion or molding, as well as during use. Esters having this general utility have been proposed and/or prepared in the past. Examples include Dexter et al U.S. Pat. No. 3,758,549, which teaches transesterification procedures for preparation of these general types of products. Included is pentaerythritol tetrakis (3-n-dodecylthiopropionate) formed by a transesterification procedure between methyl 3-n-dodecylthiopropionate and pentaerythritol. Formation of a mixed ester is not suggested.
Prior approaches such as those suggested in Kauder et al U.S. Pat. No. 4,080,364 and Nakahara et al U.S. Pat. No. 4,349,468 teach the preparation of stabilizers for polyolefins by reacting an alpha-olefin with a mercaptocarboxylic acid or an ester of a mercaptocarboxylic acid. Pentaerythritol esters thereof are also described. It is mentioned that the alpha-olefins can take the form of a mixture of alpha-olefins. Typically alpha-olefin reaction schemes such as these produce unwanted isomer byproducts that, if not removed in a separate purification step, lower the quality of the pentaerythritol ester product which is produced. Alkylthiopropionic acids prepared by approaches such as these do not typically directly produce, without special purification, an alkylthiopropionic acid which exhibits a high purity characteristic which will, when reacted with pentaerythritol or the like, form an ester composition which contains a minimum of unwanted byproducts.
Even when ester stabilizers are prepared as esters of pentaerythritol or the like that have exceptional purity, the compatibility thereof with the polymers to be stabilized therewith can be limited. While, for example, the 3-dodecylthiopropionic tetraester of pentaerythritol is solid at room temperature, whereas the 3-octylthiopropionic tetraester of pentaerythritol is liquid at room temperature, compatibility of such esters with a polymer to be stabilized is not readily predictable from this or any other physical property. Compatibility is a useful property usually defined in terms of manifestations of incompatibility which develop during blending of the stabilizer with the polymer, during storage, and/or during extrusion or other formation procedures to which the stabilized polymer is subjected, and can only be determined by experiment. A stabilizer that is especially compatible with a particular polymer or polymer blend will exhibit the absence or at least minimization of any manifestation of incompatibility such as exudation of a liquid or crystallization of a solid additive on the polymer surface. A valuable consequence is that a more compatible stabilizer can be used in greater concentrations, thus resulting in enhanced stabilizing effectiveness.
Variations in physical properties of the alkylthiopropionate tetraesters of pentaerythritol can be achieved by forming esters having alkyl groups of intermediate carbon chain lengths. For example, the 3-decylthiopropionate tetraester with pentaerythritol will tend to have properties generally midway between the dodecyl and octyl tetraesters, and the decyl ester would typically be less volatile than esters prepared from the octyl mercaptan, while being less waxy than esters prepared from the dodecyl mercaptan. However, because each ester molecule has alkyl mercaptan groups of the same carbon chain length, the tailorability of these stabilizers for achieving desired physical and compatibility properties for a specific polymer or polymer blend is limited. These properties can be varied on a somewhat gross scale, but it is not possible to more precisely vary or fine tune the compatibility properties of these types of stabilizers.
Accordingly, it would be desirable to provide stabilizing mixed alkylthiopropionic acid esters of pentaerythritol or the like which incorporate in the same molecule alkyl mercaptan groups which have at least two different alkyl carbon chain lengths. It is further desirable that these mixed esters be prepared in a manner such that the structure of the mixed ester thus produced can be controlled and can be reproduced on a substantially consistent basis. Achieving this objective includes the need to avoid the formation of any undesirable byproducts so that the mixed ester is still a high purity stabilizer product that can be reliably produced as one which provides desired compatibility properties.
By the present invention, stabilizers for extrudable thermoplastic polymers and polymer resin blends are prepared. The stabilizers are mixed esters which are tailored to exhibit specific selected properties for providing excellent compatibility with the polymer resin or polymer resin composition. Each mixed ester has at least two different alkyl groups provided by the alkyl mercaptan from which the mixed ester stabilizer is prepared. In preparing these stabilizers, preferably a 3-alkylthiopropionic acid having an alkyl group of one carbon chain length is prepared by directly reacting the desired alkyl mercaptan with an alkali metal acrylate, the reaction being carried out in the presence of strong base catalyst, followed by acidification and collection. A different 3-alkylthiopropionic acid is prepared in the same manner by using a different alkyl mercaptan having a different alkyl carbon chain length. Additional, different 3-alkylthiopropionic acids can likewise be prepared. The thus prepared different 3-alkylthiopropionic acids are reacted with pentaerythritol or the like, thereby forming the mixed ester stabilizer. When desired, the mixed ester product can be solvent refined.
It is accordingly a general object of the present invention to provide alkylthiopropionate mixed esters and an improved process for preparing same.
Another object of this invention is to provide an improved process for preparation of an improved mixed ester from at least two different S-alkylthiopropionic acids which are prepared by a direct addition reaction mechanism, as well as the mixed ester produced thereby.
Another object of the present invention is to provide improved alkyl mercaptan mixed ester stabilizers which are tailorable to achieve enhanced compatibility with any one of a wide variety of extrudable thermoplastic polymer resins or polymer resin compositions.
Another object of this invention is to provide an improved process for preparing mixed ester stabilizers which includes an improved work-up purification procedure.
Another object of the present invention is to provide a process and product produced thereby wherein a mixed ester stabilizer is prepared from a plurality of 3-alkylthiopropionic acids which are formed without requiring a substantially high excess of acid reagent, thereby minimizing the cost and inefficiency of having to remove excess acid therefrom so as to minimize or eliminate purification at an intermediate stage in the preparation of the mixed ester stabilizer.
These and other objects, features and advantages of the present invention will be clearly understood through a consideration of the following detailed description.